I have found one instance in Valmiki Ramayana, Bala Kanda: Sarga 14, where the term Hetuvada is used. This word is used during the Horse-sacrifice of Dasharatha where Sages and Brahmins were doing intellectual debates. One translator translates it as intellectual debate
कर्मान्तरे तदा विप्रा हेतुवादान्बहूनपि |
प्राहुः सुवाग्मिनो धीराः परस्परजिगीषया || १-१४-१९
- tadaa dhiiraaH vipraaH = then, intellectual, Brahmans; karmaantare = rituals in interludes; parasparajigiiSayaa = havng desire to defeat each other; bahuun hetuvaadaan = many, intellectual debates - arts of reasoning; suvaagminaH praahuH = good debaters, have debated.
Those eminent Brahmans that are good debaters have debated many intellectual debates to defeat each other, during the gap-periods of ritual works. [1-14-19]
And other translator translates it is disputation
तदा then, वाग्मिन: eloquent, धीरा: sagacious, विप्रा: brahmins, तदा then, कर्मान्तरे in between ceremonies, परस्परजिगीषया mutually desirous of victory, बहून् various, हेतुवादान् disputations, प्राहु: च engaged.
In the interval between ceremonies, eloquent and sagacious brahmins were engaged in various disputations, desirous of victory.
So, atleast in Ramayana it is not used as derogatory word.
But in Mahabharata, this word (Hetuvada) is related to atheists. In Mahabharta this word occurred multiple times. For example in Vana Parva: Markandeya-sanyasa Parva: Chapter 188
26 na vratāni cariṣyanti brāhmaṇā veda nindakāḥ
na yakṣyanti na hoṣyanti hetuvādavilobhitāḥ
And the Brahmanas, speaking disrespectfully of the Vedas, will not practise vows, and their understanding clouded by the science of disputation, they will no longer perform sacrifices and the Homa.
So, I don't think that Bhakti movement is responsible for the semantic shift of the word Hetuvada.